For example, it is reported that, when a commercial metformin hydrochloride-containing tablet and a commercial olmesartan medoxomil-containing tablet are packed together in a one-dose pack, then the metformin hydrochloride-containing tablet turns reddish. This phenomenon is assumed to be caused by the event that the (5-methyl-2-oxo-1,3-dioxol-4-yl)methyl (hereinafter referred to as “DOMDO”) group released from olmesartan medoxomil in the olmesartan medoxomil-containing tablet is hydrolyzed and converted into diacetyl and acetoin, and then these react with the guanidino group of metformin hydrochloride in the metformin hydrochloride-containing tablet. This reaction is known as Voges-Proskauel (VP) reaction. As a matter of course, however, the discoloration of drugs is unfavorable and it is considered necessary to prevent the discoloration by means of some devices in drug preparation methods.
In general, for the purpose of preventing or masking discoloration or coloration of medical and pharmaceutical tablets, uncoated tablets are coated. Depending on the properties of the compounds in uncoated tablets, the coating methods and coating agents are selected.
Recently, a polyvinyl alcohol copolymer for film-coating comprising polyvinyl alcohol (referred to hereinafter as “PVA”), acrylic acid and methyl methacrylate has been developed. This was first developed as a agent of capsules shell for solution—filling (see, for example, International Publication WO 02/17848); but owing to its excellent film formability, physical strength, adhesiveness, oxygen shieldability and the like, it has come to be applied as a film coating agent (POVACOAT (trade name)) (see, for example, PHARM TECH JAPAN (2005) vol. 21, no. 2, pp. 257-261; Proceedings of the 22th Symposium for Drug Formulation and Particle Design, pp. 77-80 (2005 in Hamamatsu).